Choo's OSE model paper does not present evidence that would allow us to say whether or not the model exhibits proactive interference or set size effects. Presumably, this is because it is specifically a model of primacy and recency effects, not a general model of working memory. The thesis scope is given on page two:
Two major behavioural effects are often observed when analyzing recall performance data. The first is known as the primacy effect, which is generally seen in most serial recall tasks. The primacy effect describes the tendency for items near the start of a sequence to be more accurately recalled than other items in the sequence. The second effects is the recency effect, which describes the opposite of the primacy effect.
Identifying the mechanisms in serial memory that are responsible for this behaviour [primacy and recency effects in serial memory] and developing a model that is able to capture this behaviour while maintaining biological plausibility is the primary motivation of the research behind this thesis.
All results are reported with respect to serial position, because the scope of the thesis is relatively narrow. Without also modeling response time and semantic categories, I'm not sure how or why the particular model would be adapted to investigate either of these effects. That's just not what it does.
Developing a model of proactive interference with Spaun seems improbable, because the framework doesn't appear to have any way to model semantic or declarative memory, and you need that to show proactive interference effects. There is no reason the Spaun framework/NEF could not be used as a basis for memory models which exhibit set-size effects, however. Spaun's input is visual and image-based, so that should be no problem.
For proactive interference, something like ACT-R, or another biologically plausible cognitive architecture which already features declarative memory, would be more apt. There are a number of papers on modeling proactive interference with ACT-R on the publications page (click on "Interference").